In recent years wireless communication networks having a plurality of base stations or other access points to provide wireless communication links pervasively throughout a composite service area have become almost ubiquitous. In providing pervasive coverage throughout a composite service area, base stations of such communications networks are often disposed in proximity such that inter-base station interference is experienced. For example, cellular base station deployment patterns used with respect to cellular telephone networks and even some metropolitan area networks (MANs) dispose base stations such that mutual interference is experienced. Such inter-cell interference can lead to unacceptably high outage rates (i.e., an unacceptably high number of communications which cannot be conducted), decreased throughput rates, decreased signal quality, etc., particularly with respect to subscriber stations disposed near the edge of a cell.
Various techniques have been employed to mitigate or avoid inter-cell interference. For example, frequency reuse patterns (e.g., frequency reuse of 7, wherein only every 7th cell in the cellular deployment pattern may use a particular frequency) have been developed so as to separate the cells in which any particular frequency is reused by sufficient distances, thereby resulting in attenuation of inter-cell signals for interference mitigation. However, such frequency reuse patterns result in substantial spectral inefficiency. For example, in the foregoing frequency reuse of 7 example, the frequency reuse pattern results in a 86% decrease in the available spectrum at any particular base station.
Another technique that has been used to mitigate inter-cell interference is downlink power control. However, appreciable feedback overhead is utilized in providing information for downlink power control from the subscriber stations to their respective base station. Moreover, such downlink power control not only reduces the signal energy and thus the interference in a neighboring cell, but it also reduces the energy of the signal as received by an intended subscriber station. Accordingly, many subscriber stations, such as those disposed near the edge of a cell, those in a shadow or signal fade, etc., may experience outages, decreased throughput, decreased signal quality, etc.
Interference cancellation is yet another technique that has been used to mitigate inter-cell interference. Subscriber stations have been adapted to include sophisticated circuitry which actively cancels interfering signals from desired signals in the signal as received by the subscriber station. However, such circuitry is typically complicated, requiring appreciable knowledge with respect to the interfering signal and/or desired signal for effective cancellation. Accordingly, such circuitry is often relatively expensive, increasing the cost and complexity of subscriber stations appreciably.
Where base stations have been equipped with multiple antennas, downlink beam forming has been used to mitigate multi-cell interference. For example, highly directional antenna beams have been used to direct signal energy to a single selected subscriber station, thereby minimizing signal energy propagating into neighboring cells. However, such downlink beamforming typically requires either feedback of channel state information (CSI) or an assumption that the downlink has the identical channel as the uplink and the radio frequency (RF) front end is calibrated to compensate for the mismatch between the uplink transmitting and downlink receiving circuits. Feedback of CSI generally requires large overhead signaling, and thus results in spectral inefficiency. Assuming that the downlink has the identical channel as the uplink (e.g., to avoid downlink CSI feedback by measuring uplink CSI at the base station) is often not accurate, particularly in frequency division multiplex (FDM) systems and even time division multiplex (TDM) systems where the subscriber stations are highly mobile or fast moving. Calibration of RF front end circuitry to compensate for the mismatch between uplink transmitting and downlink receiving circuits is relatively complicated, resulting in appreciably increased implementation and deployment costs.
Although not utilized for avoiding or mitigating inter-cell interference, a technique for implementing spatial division multiple access (SDMA) at a base station has opportunistically relied upon random beam configurations, which are not formed to address channel characteristics experienced by any particular subscriber station, and the random distribution of subscriber stations to provide SDMA grouping and SDMA resource assignment with respect to subscriber stations. For example, the base station may form a plurality of random beams, each subscriber station in that base station's service area may measure the signal quality with respect to each of the random beams and report a best beam to the base station, and the base station may assign this beam (assuming the beam is not already assigned to another subscriber station for the same frequency resource) to the subscriber station. Other subscriber stations in the base station's service area may similarly be assigned different random beams, although each using a same frequency resource assignment.